Pressure sensitive adhesives and adhesive-coated sheet materials are well-known in the art as having the ability to adhere to adhesive-receptive surfaces on mere contact. Such coated sheets in the form of labels and tapes, for example, can be adhered to various substrates under very light "finger pressure" and can later be removed therefrom by stripping the sheet from the surface to which it is attached. It is often desirable to reduce the initial affinity of a PSA for a substrate while still maintaining sufficient ultimate adhesion (after pressure bonding). The prior art has attempted to achieve this result in various ways, mainly by use of different types of "physical spacers" which have been mixed with or coated on the surface of the PSA. Such spacers act to hold the adhesive slightly away from the substrate surface until the desired placement has been achieved. For example, U.S. Pat. Nos. 3,314,838, 3,331,729, and 3,413,168, all assigned to the present assignee, disclose the use of hollow, spheroidal particles or microballoons composed of glass, urea- or phenol-formaldehyde resins, etc., to decrease initial adhesion and thereby provide positionability. The microballoons are described as being crushable or collapsible under applied hand pressure, thereafter enabling the PSA to contact and adhere to the surface of the substrate. Fragments of the collapsed or crushed microballoons may, however, remain on the bonding surface of the adhesive rather than completely dispersed into the adhesive layer, and this may, at least initially, adversely affect the adhesion strength.
Silicone has also been used as a spacing material to reduce the initial adhesion of PSA coatings and thereby produce positionable products. For example, U.S. Pat. No. 3,554,835 (Morgan Adhesives Company) discloses a method of making and using a "slidable" PSA-coated laminate which relies upon "dots" of silicone or other conventional release material as non-adhesive spacers to enable initial positioning of the laminate on a substrate. Similarly, United Kingdom Pat. No. 1,541,311 (United Merchants and Manufacturers, Inc.) describes a PSA-coated laminate where positionability is provided by a uniform and discontinuous surface coating in the form of small beads or droplets of a non-tacky polysiloxane or polysiloxane-polyoxyalkylene block copolymer. Both teachings require an additional manufacturing step or steps to apply the silicone spacing material and to then effect solvent removal, drying, or curing.
In U.S. Pat. No. 4,151,319 (United Merchants and Manufacturers, Inc.) a method for making a positionable PSA-coated laminate is disclosed whereby polysiloxane or polysiloxane-polyoxyalkylene block copolymer is intimately mixed with the PSA itself rather than forming a coating on the PSA surface. "Pre-mixing" of a siloxane polymer or copolymer in a suitable solvent prior to incorporation into the adhesive is recommended. Again, the required mixing or dispersing and the recommended pre-mixing contribute additional process steps. The siloxane additives of this teaching are said to be in the form of small beads or droplets dispersed throughout the adhesive mass, and thus some of the droplets are positioned at the adhesive/release sheet interface of the laminate (i.e., at the PSA surface) and function to allow initial positioning on a substrate. The use of either block copolymers or high molecular weight (above 25,000) polysiloxanes is suggested in United Kingdom Pat. No. 1,541,311 and U.S. Pat. No. 4,151,319 to minimize loss of adhesive mass, but the resultant laminates still may not offer predictable levels of positionability even with minor losses.
U.S. Pat. No. 4,346,189 (Morgan Adhesives Company) describes the use of polysiloxane additives (of up to about 10,000 molecular weight) in a different type of application. The silicones are mixed with tackified, synthetic rubber-based PSA compositions to reduce edge ooze or flow upon cutting of sheets coated with such compositions. Either non-reactive or reactive polysiloxanes can be utilized and are said to appear to adsorb or graft onto other ingredients of the adhesives or take other unknown actions so as to produce the desired effect. It is stated, however, that the silicones can be added even at relatively high concentrations (6 to 10% solids) without adversely affecting the adhesive characteristics of the material.
Graft copolymers, some containing silicone, are being increasingly used for general modification of surface properties, as is described in a series of papers by Y. Yamashita et al., [Polymer Bulletin 7, 289 (1982); Polymer Bulletin 10, 368 (1983); Makromol. Chem. 185, 9 (1984); Macromolecules 18, 580 (1985)]. Such use is also reflected in some recent Japanese art, such as Japanese Pat. Application No. 57-179246, published Nov. 4, 1982, which concerns the use of graft copolymers as additives to impart long-lasting hydrophobicity (or hydrophilicity) to surfaces. In Japanese Pat. Applications Nos. 58-167606, published Oct. 3, 1983, and 58-154766, published Sept. 14, 1983, a method of preparation of silicone graft copolymers and the use of these copolymers in coating compositions such as paint films are described. Here, the copolymers are said to provide long-lasting water- and oil-repellency, stain resistance, and reduced frictional properties. Japanese Pat. Application No. 59-78236, published May 7, 1984, discloses a method of preparing monofunctional polymeric silicone monomers, i.e., macromonomers, for use in the preparation of graft copolymers as surface-treatment agents. The use of such silicone macromonomer-grafted copolymers in coating compositions, again to impart lasting water- and oil-repellency, stain resistance, and low friction characteristics, is described in Japanese Pat. Application No. 59-126478, published July 21, 1984. U.S. Pat. No. 4,728,571 discloses the use of tack-free polysiloxane-grafted copolymers (and blends thereof with other polymeric materials) as release coating compositions for PSA-coated sheet materials and the back side of PSA-coated tapes.
U.S. Pat. No. 4,693,935 teaches polysiloxane-grafted copolymer pressure sensitive adhesive compositions and sheet materials coated therewith. The invention is directed specifically towards pressure sensitive adhesives and articles coated therewith.